One type of emission tomography device is an ECT (Emission Computed Tomography) device, which will be explained using a PET (Positron Emission Tomography) device as a particular example. In a PET device, the two gamma rays that are produced through the annihilation of a positron are detected by a plurality of detectors. Specifically, a radiopharmaceutical that includes a positron-emitting isotope is injected into the patient body, and the pair of annihilation gamma rays that are emitted from the patient body that has been injected are detected by a large number of emission detectors. Given this, if gamma rays are detected within a given period of time by two emission detectors this is considered to be simultaneous detection, and is counted as a pair of pair annihilation gamma rays. The location at which the pair annihilation occurred is specified as being on a straight line connecting the locations of the two emission detectors that detected the radiation. In this way, simultaneous count information is accumulated and a reconstructing process is performed to produce an image of the positron-emitting isotope distribution, that is, to produce a tomographic image.
An emission detector that is used in a PET device is a combination of a scintillator block, which has a plurality of scintillators, and a photon multiplier tube (PMT). Many emission detectors are arranged in the form of a ring to structure a detector ring. Data collection is conducted by performing simultaneous counting of pair annihilation gamma rays by the emission detectors arranged in this way. In order to evaluate efficiently the simultaneous counts of events detected by the individual emission detectors, the individual emission detectors are divided into a number of groups, such as disclosed in Japanese Unexamined Patent Application Publication H6-342074. The identification of simultaneous counts between individual groups is performed by compiling event information for the individual emission detectors within each group, with each group, which has a plurality of emission detectors, functioning as a single emission detector. Furthermore, as disclosed in Japanese Unexamined Patent Application Publication 2011-232044, there is also a Time of Flight (TOF)-type PET device able to specify the location at which the annihilation emission pair was produced using the time difference in the annihilation emission pair in a PET device.
The evaluation of the simultaneous count between groups will be explained in reference to FIG. 9. Respective detector signal processing circuits 72 are connected to the emission detectors 71 that detect incident gamma rays as pulse signals. A detector signal processing circuit 72 has a location calculating circuit 73, an energy calculating circuit 74, and a timing circuit 75 for detecting the respective location information for incident gamma rays, energy information for incident gamma rays, and detection timing information, based on pulse signals inputted from the emission detectors 71.
In an emission detector 71, the location information, energy information, and timing information for an event wherein a gamma ray is detected as a pulse signal are sent, as single event information, from the detector signal processing circuit 72 to the grouping circuit 76. The grouping circuit 76 sequentially outputs, to a simultaneous count circuit 77, the event information that is sent from the plurality of detector signal processing circuit 72. The simultaneous count circuit 77 uses the timing information that is included in the event information to perform identification of simultaneous counts between grouping circuits 76. In this way, the event information is sent from the detector signal processing circuits 72 through the grouping circuits 76 to the simultaneous count circuit 77.